[Haskell-cafe] GADT and problems with rigid type variables

[Daniel Peebles]

The problem is that you have an existential `t` there, and two values of the
type Foo might not have the same `t` inside them.

What do you want to happen if someone writes Foo True == Foo "yep"?

The only real solution here is to parametrize your Foo type by the t that
lives within it, so you can ensure the inner types are the same. You could
also do some (in my opinion) fairly nasty stuff with Dynamic or Typeable,
adding a constraint to the Eq and attempting to cast at runtime (returning
False if the cast returns Nothing).

Hope this helps!

On Mon, Aug 23, 2010 at 12:36 AM, Markus Barenhoff <alios at alios.org> wrote:

> Hello,
> playing with GADTs I ran into a problem with rigid type variables
> which is ilustrated by the following example. I think it should be
> pretty clear what I'am trying to express... Any suggestions?
>
> ---- snip ----
> {-# LANGUAGE GADTs #-}
>
> data Foo where
>  Foo :: (Eq t) => t -> Foo
>
> instance Eq Foo where
>  (Foo a) == (Foo b) = a == b
>
> {-
> Scratch.hs:7:28:
>    Couldn't match expected type `t' against inferred type `t1'
>      `t' is a rigid type variable bound by
>          the constructor `Foo' at /home/alios/src/lab/Scratch.hs:7:3
>      `t1' is a rigid type variable bound by
>           the constructor `Foo' at /home/alios/src/lab/Scratch.hs:7:14
>    In the second argument of `(==)', namely `b'
>    In the expression: a == b
>    In the definition of `==': (Foo a) == (Foo b) = a == b
> Failed, modules loaded: none.
> -}
> ---- snip ----
>
> thnx
> Markus
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20100822/489c9a27/attachment.html